drm/amdgpu: fix typo in dce10 watermark setup

Using the wrong watermwark value for the secondary
watermark.  Copy paste typo.  Noticed by Mykola.

Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org